Electrochemical Preparation Of Metal-organic Frameworks Cu(Ⅱ)-Terephthalic Acid And Cu(Ⅱ)-2-Amino-Terephthalic Acid And Study On Their Electrocatalytic Activities

Metal-organic frameworks（MOFs）,as a kind of porous nano-structured functional materials,with their diverse structures,adjustable pore diameters,and high specific surface area,have great potential in gas storage and separation,catalysis,sensing,drug delivery,optics,magnetism and other applications.The electrochemical synthesis is faster,easier to control and realize continuous production,and under milder conditions than other methods for MOFs preparation.Therefore,we synthesized electrochemically two copper-based MOFs,explained the mechanism of MOF film formation,explored the influence of experimental parameters on the crystal size and film density of the prepared MOFs.The structures and performances of the prepared MOF films were characterized by cyclic voltammetry（CV）,scanning electron microscope（SEM）,X-ray diffraction（XRD）,and Fourier infrared（FT-IR）spectroscopy.And we tested their performances as electrochemical sensors for low concentration malachite green（MG）.The result shows that（1）In a three-electrode system with platinum wire as the counter electrode,saturated calomel electrode as the reference electrode,and copper chip as the working electrode,terephthalic acid（H₂BDC）and 2-amino-terephthalic acid（H₂BDC-NH₂）are used as ligands,respectively,and copper chip as the metal ion source and potassium chloride are the supporting electrolyte,and the MOF films Cu-BDC and Cu-BDC-NH₂were synthesized,respectively,by anodic oxidation in a p H9 ammonia solution at room temperature.The influence of various experimental parameters on film formation was studied.The experimental results show that for Cu-BDC,a dense Cu-BDC film can be obtained in the presence of the ligand of 0.05mol/L and in the potential range of0.7～1.1V for 2 hours.In the potential range of 0.7 to 1.1V the density of the film layer increases with the potential.At a potential of 1.0V the density of the film increases with the decrease of the ligand concentration in the range of the ligand of 0.05 to 0.1 mol/L.For Cu-BDC-NH₂,a compact Cu-BDC-NH₂ film can be obtained in the presence of the ligand of 0.05mol/L and in the potential range of 2～3.5V for 2 hours.In the presence of the ligand of 0.05mol/L and in the potential range of 2～3.5V the density of the film layer increases with the potential.At a potential of 2.5V,the density of the film increases with the increase in the ligand concentration in the range of 0.05 to 0.1 mol/L.Both the intactness and density of the two films increase with the increase of the reaction time.（2）Based on the fact that the peak potential decreases gradually from the beginning to the end of the Cu-BDC film formation,it is proposed that the formation of Cu-BDC film is the result from BDC^2-gradually replaces the ammonia of complex cation,[Cu（NH₃）₄]²⁺formed rapidly from anodically formed Cu（II）with NH₃ in an aqueous solution containing NH₃,leading to the reduction peak potential continued to decrease as ammonia was grandually replaced by terephthalic acid because terephthalic acid is a stronger ligand of Cu（II）than ammonia.That may make the film composition different at different stage.It is expected that the electrochemical synthesis can be used not only as a means of MOF film preparation,but also for real-time monitoring of the process.（3）Cu-BDC films prepared for 2h at a constant potential of 1V in the presence of the ligand of 0.05 mol/L and Cu-BDC-NH₂ films prepared for 2h at a constant potential of 2.5V in the presence of the ligand of 0.05 mol/L Characterized were characterized.The scanning electron microscopys shows that Cu-BDC is a rhombohedron with a particle size of about 180nm,and Cu-BDC-NH₂ is a cube with a particle size of about200～600nm.Compared with the XRD and FT-IR results in the literature,Cu-BDC and Cu-BDC-NH₂ have been successfully synthesized.Among them,Cu-BDC bridges the dinuclear Cu（II）with the ligand carboxyl group to form a four-blade paddle wheel-shaped structure secondary structure unit as a grid plane network of nodes,and the plane layers rely on weak interaction（the ligand benzene ring plane The stacking）overlap each other to form a three-dimensional structure.Thermogravimetric and differential thermal analysis shows that both Cu-BDC and Cu-BDC-NH₂ have good thermal stability.（4）Carbon paste electrodes（CPEs）were modified,respectively,with as-prepared Cu-BDC and Cu-BDC-NH₂ from the films into Cu-BDC/CPE and Cu-BDC-NH₂/CPE and they were explored for malachite green（MG）sensing.The results from CV experiments show that both Cu-BDC and Cu-BDC-NH₂ have significant electrocatalytic activity for MG oxidation,and can be used potentially as electrocatalysts for MG oxidation degradation and electrochemical sensors for MG detection.And the electrocatalytic activity of Cu-BDC-NH₂/CPE is significantly higher than that of Cu-BDC/CPE.The relationship between the CV scanning rate and the reduction peak current of the prepared electrode indicated that MG oxidation is a surface diffusion-controlled electron transfer process.（5）The differential pulse voltammetry（DPV）was used to study the sensing performance of Cu-BDC/CPE and Cu-BDC-NH₂/CPE on MG,respectively,and the linear relationship between the reduction peak current,Ipa,and the concentration,C of malachite green was obtained:(Ipa=11.282C+41.177,R²=0.9915,and(Ipa=9.4864C+119.53,R²=0.9162.And the detection limit is 3.9×10^-9mol/L for Cu-BDC/CPE.It is expected to apply it in the detection of MG in environmental water.